Nano-indentation by atomic force microscopy (AFM) has the sensitivity and resolution to provide needed data on regional mechanical properties of living cells. However, interpretation of the results is complicated by the pyramidal shape of the AFM probe tip, and its small size relative to the depth of indentation. To help realize the full potential of this technology, the overall goal of this proposal is to perform a thorough engineering analysis of the finite-indentation problem, emphasizing the effects of relative indentation depth and tip geometry, and use the findings to guide AFM indentation studies on the mechanical properties of living cells. A systematic series of finite element simulations will be used to specify the conditions for which popular simplistic methods for AFM analysis are valid, and to address two other specific aims testing whether indentations can be used to identify material heterogeneity, non- linearity, and anisotropy. A fourth specific aim utilizes simultaneous static stretch and AFM identification of cells to test the hypothesis that resting cardiac fibroblasts have non-linear and isotropic material properties, but they become anisotropic after reorientation of actin stress fibers in response to cyclic stretch.